In matrixed four channel audio systems, two channels of sound are obtained from a phonograph record or the like. The two channels of sound are generally referred to as left and right channels and are converted by an electronic matrix to left front, left back, right front and right back channels. The content of the four channels derived from the two signal channels depends on the method of matrixing chosen. In any case it is not possible to recover the four separate audio channels which were encoded as two channels for the purpose of transmission or permanent record in a primarily two channel system. Some of the four channel effect can be recovered if the four audio signals are matrixed from four to two channels in the record or transmission process and by complementary de-matrix recovered in the receiving process. It is inherent that associated with the matrixing technique incomplete separation occurs and cross talk between the four recovered channels is present at the output of the matrix. The type of cross talk that occurs can be chosen by adjustment of the encode and decode matrix.
In the system to which the invention is directed the matrixes are chosen to provide good separation between the audio signals representing left and right portions of the sound field, but limited separation between the audio signals representing front and back, this method of matrixing has been devised by the CBS Corporation and has been given the name SQ (registered trademark) by its innovators.
This invention is directed toward improving the front to back separation in a matrixed audio sound field of the SQ type. To improve the apparent separation of the front and back speakers a system has been devised whereby logic circuitry is connected to the four channels at the output of the matrix to determine which of the two pairs of channels (left front -- right front or left back -- right back) has the dominant signal thereon. The logic circuitry then uses this information to produce a pair of signals which control the gain of four amplifiers, one in each channel. The gain of the amplifiers is controlled so that the total volume remains constant but the volume of the front speakers is increased and the back speakers is decreased when the left front and/or right front channels have dominating signals therein and the opposite occurs when the left back and/or right back channels have dominating signals therein.